Urethane prepolymers which are produced from such monocyclic isocyanates as 2,4-tolylene diisocyanate; 2,6-tolylene diisocyanate and mixtures thereof are normally cured with aromatic diamines such as methylenebis (0-chloroaniline); 2,6-dichloro-p-phenylenediamine; 1,2-bis(2-aminophenylthio)ethane and the like. Attempts to cure such prepolymers with diol curatives result in the production of polymers having melting points which are too low and poor physical properties.
Urethane prepolymers which are produced from dicyclic isocyanates such as 4,4'-methylenebis (phenylisocyanate) cannot readily be cured with the above-mentioned aromatic diamines because the curing reaction in such a system is too rapid, i.e. the pot life of the polyurethane generally is measured in seconds. Diols have proven to be effective curatives for these prepolymers, however, most diols such as 1,4-butanediol; 1,6-hexanediol; cyclohexanediol; cyclohexanedimethanol etc. produce polyurethanes having reduced hardness in the polymers containing 4-7% NCO. Using these diol curatives, a prepolymer having an NCO content of 8-10% is needed to produce polymers having a hardness of 40-45 Shore D. One commercially available diol curative which results in a reasonably good cure is p-di(hydroxyethoxy)benzene.
Normally, polyurethane elastomers are made by mixing the appropriate isocyanate-terminated prepolymer with the appropriate curative at about 100.degree. C., a temperature at which the prepolymer is fluid and the curative is molten. Reaction begins immediately on manual mixing, and a pot life (the time before the mixture reaches a highly viscous state) of 3-10 minutes is generally desirable so that the mixture can be degassed and poured into a mold. If machine mixing is employed, the pot life can normally be less. Many hours may be needed for the --NCO, --OH (or NH.sub.2) reaction to be completed once the mold is filled, however, but, in practice, the material is demolded as soon as it has acquired sufficient strength to resist breaking, cracking and/or deformation. The demolded material is then "post cured" in an oven, for example, at about 100.degree. C. for about 16 hours, to complete the reaction. Thus, to be economical, the shorter the demold time, the better. The strength of the polymer at demold (while hot) is called the "green strength".
A major disadvantage of the use of p-di(hydroxyethoxy)benzene with the methylenebis(phenyl isocyanate) prepolymers, as compared to the methylenebis(0-chloroaniline)tolylene diisocyanate prepolymer systems is its much weaker "green strength" on demolding. A methylenebis(0-chloroaniline) cured polymer can often be demolded after 20-40 minutes, whereas a p-di(hydroxyethoxy)benzene cured polymer often requires 60-150 minutes to demold.
"Green strength" is, in part, related to the reaction or chain extension rate, and the reaction can be shortened by the use of a catalyst. p-Di(hydroxyethoxy)benzene curing can be advanced to less than 60 minutes demold time, however, by the use of an amine or tin catalysts and still retain an adequate pot life of 4-5 minutes. Further shortening of the demold time reduces the pot life to a point where it is no longer practical.
"Green strength" is a qualitative term and no standard method is available for assessing it. In the present invention, the following definition is used:
The mold is opened and a piece of flashing from the specimen (80-100 mil thick) is folded back against itself immediately, while hot. Ratings are determined as follows:
Poor--Breaks easily, without much bending. PA0 Fair--Breaks when folded back to about 1/8" of touching itself. PA0 Good--Doesn't break when folded back to about 1/8" of touching itself. PA0 Very good--Breaks only when pressure is applied at the fold. PA0 Excellent--Doesn't break even when pressure is applied at the fold.